Can you place a limit order after hours (Facebook) and does the time matter?

I've placed lots of limit orders but never when the market wasn't open.  Can I place a limit order for 1 facebook share right now, 4pm 12am, etc?  

Does the time the limit order was placed matter?  For example you place a limit order of $45 right now and I place one at 7am.  Does yours get filled before mine (if it is filled).

Thanks.

Merrill Lynch is who I have been using recently.  So if two people placed trades with say etrade at 5pm today and 7am tomorrow morning they may go in order and they may not?  

Is there anything stopping anyone from putting in the limit orders right now.